HV battery balancing

excellent! thanks scotman. i'm going to look into the scangauge 2. i really appreciate your help.

 

That's what we are here for. Just trying to do my part to help save you some money and keep your prius on the road.


Your welcome

 

No problem slim

 

here's my first attempt with a genius g 750 smart charger. i was afraid of stringing all 38 together in parallel so yesterday i experimented with doing one at a time and then i noticed my charger didn't recognize the low reading modules (0.26, 0.14, etc.) so i started charging 8 or 10 modules together in parallel, mixing in some of the higher reading modules so my charger would kick in. my charger showed the charge cycle complete (red light turned to green) after only about 15 mins. i took a reading and the levels went up. they also seemed to come down just as fast. no surprise since this hv pack is from a 2001 that was probably sitting dead for over a year.

since i am just experimenting and trying to learn a few things i decided to string up all 38 modules in parallel this evening. hopefully i'll find some good 6.8+ modules that will withstand a load so i can put them in another pack.

 

Slim I see you charging the batteries without them being clamped.

I realize in the pictures the modules are all in series, but it is still possible for one module to be taking a large current "especially if it has one or more shorted cells" this could cause it to swell and burst.

It is always advisable to have a piece timber at each end of the pack clamped by a couple of "G" clamps to stop the swelling.

John (Britprius)

 

ahhh. that's right john. i remember reading that earlier in the post. i will buy some long clamps for this next time. i took off the charger last night and they were all reading 6.98 as a whole parallel block of 38. i'm going to take the individual voltage readings later today after i have them sit. should i have removed my 2 long parallel + and - wires immediately after charging or is it ok to keep the wires bolted in? i was thinking maybe i'll have to charge a couple of times? or is charging once enough to see which modules are good? thanks again!

 

If you are charging NiMH batteries in parallel unattended be extra careful, leave plenty of space around the work area, and make sure your fire insurance premium is paid up. I'm serious.

 

The ideal method of charging is to charge modules independently with a charger such as the "Supermate DC6" this can be set to charge and discharge the modules and record the amp/hr capacity.

The idea is that each module contains 6cells, "a battery within a battery" some maybe half charged others fully charged, but the maximum amp/hr capacity of the module is that of the lowest cell. If you carry on past this point that cell will be damaged by reverse charging.

If you slightly over charge the module the cells that are fully charged and are continued to be charged will give this over charge off as heat. The cells that are only partly charged will continue to be charged and catch up increasing the capacity of the module.

Over charging slightly and then discharging three times recording the amp/hr capacity on each discharge will show any improvement in a module, "balancing the cells".

When you have sufficient modules balanced you can choose the modules with the most even amp/hr capacity. It is better to have a full pack of 4.5 amp/hr modules than a mixed pack of 5 and 6 amp/hr modules. The reason again is that the maximum amp/hr capacity of the complete battery is that of the lowest module, and any discharge past this point will reverse charge that module destroying it, and rendering the rest of the battery useless.

When you have chosen your modules you should charge them all to the same voltage (independently) then connect in parallel and let sit for some hours to completely even up, then build your battery.

The Suppermate DC6 is designed to charge and balance NIMH battery packs for electric model planes, cars etc, so is perfect for balancing the Prius battery modules. More than one of these chargers is useful.

John (Britprius)

 

The problem with charging the modules in parallel, "apart from the safety aspect" is that the total capacity of the 38 modules is about 230 amp/hrs so charging at 5amps assuming it is 100% efficient would require 46 hours of charge.

One module with a shorted cell would stop all the others charging by loading the charging voltage to 6 volts and overheating that module.

It is also a very good idea to have a cooling airflow over modules when being charged.

John (Britprius)

 

John, are you sure. I mean in the second picture from the top I see a bass drum behind the battery, if he connects that to the battery it could keep going and going and going and going.

 

first off you guys are awesome, and way ahead of me with this stuff; that is for sure. i'm a musician and got into buying and selling priuses to make extra cash. now that i've done almost 20 cars i see the need to get my head around these modules and the rebuilding instead of buying salvage packs for $1000-1200 each time. i see now that i need the proper charger as mine turns itself off automatically when it thinks the modules are fully charged and it doesn't show the amp hrs. which seems to be crucial. and paul, i'll take what you said seriously. maybe it's good that i only have this crummy smart charger as it won't let me over-charge. will a module explode or catch fire or both? i'm doing this inside my recording studio/apartment and my wife will kill me if i catch the place on fire. for now i'm going to study and try to understand all the information john and everyone has given me. thanks fellas!

 

Slim the batteries have a safety pressure relief valve but this operates at a pressure higher than the sides can withstand without support, hence the need to clamp the modules while being charged. As you probably have noticed they are clamped as a pack when installed in the battery case.

While the liquid in the batteries is not flammable the plastic cases are, but the main problems with fire are short circuits.

When the 38 modules are in parallel the short circuit current will easily be over 4000 amps "enough to vaporize screw drivers and other metal tools".

In series as in the assembled battery once a short has taken place you have 100+ amps at around 275 volts or
27.5 kwatts plenty to cause a fire.

Paul and I are not trying to be alarmist, but please work safely and methodically also bearing in mind the voltage of a completed battery can give a lethal bite.

John (Britprius).

 

i did see a few sparks flying when i attached my parallel wire going across all 38 cells. vaporizing a screw driver is NO joke! how do i stay away from the short circuit current while screwing into place my parallel wires? maybe i should just stick to swapping out gen 1 modules with gen 2 modules. i'm getting a little nervous now but was really hoping to figure this out.

 

Nervous is good it makes you think.

The high current is from one end of the module to the other. It is taking liberties with temporary connections from the charger or trying to connect a module the wrong way round to a pack of others in parallel, and that sort of thing.

Charging one module on it's own well clamped is not really a problem but charging a pack in parallel and have the middle one blow up like a balloon pushing modules in every direction could be.

Do not let this put you off just be careful and work methodically. I would be foolish not to point out the dangers, "like telling you not to smoke when filling your car with fuel".

One further point do not try to rush the charging of modules by charging at a high rate on the work bench. I like to keep it under 1 amp per module 500 ma is even better especially when balancing modules as remember any cells at full charge will give off the excess charge as heat, and to much heat will dry the cells out.

John (Britprius).

 

thanks again john! that gives me plenty to think about. my charger doesn't give me any of the options you are mentioning. i can switch from 6 volts or 12 volts and that's it and it turns off automatically. i'm gonna try to find a dc6 or the mrc 989 and then at least i know i'll have the right tools to do this properly. as of now my 38 modules which are still strung up in parallel (as in my pictures) are reading 5.99 since sitting for 12 hrs. but this seems pointless now since i can't balance and read the amp/hrs like you were saying so... i may have to hold off untill i get the proper charger.

 

New owner here. When should I consider balancing my battery? Perhaps after 100,000 miles? Any reason I can't install a harness and slow-charge on a regular basis while battery pack remains in the vehicle?

 

Yes the charger you have there is of little use for charging even one module, as the voltage required would need to be over 7.2 volts to achieve any charge but needs to put out in the region of 8.5 volts to fully charge a module.

That charger will only give out 750 ma or 3/4 of one amp so if it was connected to 38 modules in parallel and gave out 8.5 volts it would only take 304 hours to charge them. As stated before however charging in parallel is not a good idea.

John (Britprius)

 

ok john, i'll continue with this project after i get the proper charger. you've been quite the help and i appreciate it greatly.

 

[q

hello modelman! what we are discussing here is balancing the low voltage or failed modules inside of your hv battery pack after it has failed. this is done in hopes of bringing the modules back to life so they can be used to rebuild your hv battery. (yes, the one that toyota will charge you $4200 to install) hopefully you won't have to do this anytime soon with your 2013.

 

Balancing is something I have contemplated, but charging in the vehicle is not a good idea since the Prius battery is never fully charged or discharged by the vehicle.

The reason for this is to prolong the life of the battery. Fully charging and or discharging a NIMH battery on a regular basis drastically shortens it's life.

Balancing the battery with one charger would take about one month so not a quick job with the car off the road.

This may have some benefit if at the same time modules were moved center to outside to reduce the effect of drying out in the warmer positions in the center of the battery.

I do not know anyone that has tried this and any gain would be difficult to quantify.

John (Britprius)